The present invention relates to an improved borosilicate glass and, more particularly, borosilicate glass having a high transmission for electromagnetic radiation in the UV region, a small thermal expansion coefficient and a high chemical stability. It also relates to a method of making the improved borosilicate glass and a method of using it.
Glasses having high UV transparency have numerous applications. They are used, for example, as EPROM windows and UV glasses, as windows in photomultipliers and spectrophotometers, and as protective tubes of UV lamps in UV reactors.
UV oxidation reactors are used to decrease or eliminate pollutants from chemically and/or biologically contaminated water in the most varied manner or for sterilizing drinking water.
The oxidative degradation of chemical compounds such as chlorinated hydrocarbons, CFCs, AOX or BTX and the oxidative destruction of bacteria in water is promoted by UV radiation--principally at a wavelength 253.7 nm. A UV radiation protection tube, which can be universally used in all UV oxidation reactor types, must therefore, in addition to high UV transmission at 254 nm, have very good chemical stability since it is exposed to attack by aqueous solutions over a very long period and possibly at elevated temperature. Since the UV oxidation reactors can also be exposed to high temperature stresses in operation, the radiation protection tube must have as high as possible a resistance to temperature change which can be achieved by low thermal expansion.
Quartz glass is highly suitable for use per se as a UV-transparent glass, but because of its high price and processing difficulties it is only used in exceptional cases requiring particularly good hydrolytic properties. A further disadvantage with quartz glass is its poor fusibility to ceramic substrates (e.g. Al.sub.2 O.sub.3), Ni--Fe--Co-alloys or molybdenum, because of a thermal expansion coefficient which is too small.
"Borosilicate glass 3.3", a borosilicate glass having minimum thermal expansion--with the highest possible chemical stability--and thus high resistance to temperature change is currently known and marketed.
Because of its small thermal expansion coefficient, .alpha..sub.20/300, of 3.3.times.10.sup.-6 K.sup.-1 and its high chemical stability, this glass has assumed an outstanding position internationally, e.g. in the areas of laboratory equipment, chemical apparatus, piping, fittings and household glass.
It is traded under various brand names--such as Duran, Pyrex, Rasotherm and Simax and has a very similar chemical composition which is approximately (% by weight) SiO.sub.2, 80 to 81%; B.sub.2 O.sub.3, 12 to 13%; Al.sub.2 O.sub.3, 2 to 3% and Na.sub.2 O+K.sub.2 O, 4 to 5%.
For Duran.RTM., according to Laboratory Glass Catalog No. 50020/1991, the composition (in % by weight) below is given by the manufacturer as: SiO.sub.2, 81%; B.sub.2 O.sub.3, 13%; Al.sub.2 O.sub.3, 2%; and Na.sub.2 O+K.sub.2 O, 4%.
However, "Borosilicate glass 3.3", because of its composition, has an insufficient UV transparency in the UV-B range of 280 to 315 nm. For wavelengths below approximately 300 nm, it is insufficiently transparent, even if the glass is produced with raw materials extremely low in Fe.sub.2 O.sub.3.
In the past, it has apparently not been recognized in developmental work involving "Borosilicate glass 3.3" that it is possible to influence its structure-related insufficient UV transparency decisively by a specific structural change.
Thus, German Patent DE-C 767,476 long ago disclosed a borosilicate glass having the following composition: (in % by weight): from 72.5 to 83% SiO.sub.2 +Al.sub.2 O.sub.3 ; from 0 to 15% B.sub.2 O.sub.3 ; from 4 to 16% alkali metal oxides and alkaline earth metal oxides.
From U.S. Pat. No. 3,258,352, those skilled in the art know of a glass having the following composition (in mol. -%)&gt;75% SiO.sub.2 +B.sub.2 O.sub.3 +Al.sub.2 O.sub.3 ; from 0 to 20% Al.sub.2 O.sub.3 ; from 0 to 50% B.sub.2 O.sub.3 ; from 0 to 20% Li.sub.2 O+Na.sub.2 O+K.sub.2 O; from 0 to 30% MgO+CaO+SrO+BaO; and from 0 to 20% PbO.
JP-B 92/33741 B2 teaches a glass having the following composition (in % by weight) from 68 to 82% SiO.sub.2 ; from 0.5 to 5% Al.sub.2 O.sub.3 ; from 10 to 18% B.sub.2 O.sub.3 ; from 3.5 to 8% Li.sub.2 O+Na.sub.2 O+K.sub.2 O; from 0 to 3% CaO+MgO; and from 0.05 to 1% fining agent.
It cannot be inferred from the prior art that there is a borosilicate glass of the type "Borosilicate glass 3.3" having a high transmission in the UV region.
The only highly UV-transparent material with low thermal expansion and high chemical stability currently known is quartz glass or silica glass.
However, this single-component glass, as already mentioned at the outset, is very expensive and can be produced only with difficulty or not at all in complicated geometric dimensions and narrow tolerances.